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Question

I have a simple question, i am new to Haskell and trying to learn a lot. How can i concatenate for example

 [( (0,1) , [1,2,3]) , ((1,2) , [7,8,9]) ,((0,1) , [4,5,6]) ] to [[1,2,3,4,5,6] , [7,8,9]]

I want to concatenate the lists that have first element of the pair equal. I tried

 map concat list 

 map (\t -> (filter (==).first) list ) list 

but the first one doesn't work and the second one gives an error. Any ideas?

Answer 1

using a couple functions from Data.List and Data.Function.on this becomes:

import Data.List     (sortBy, groupBy)
import Data.Function (on)

sortByFirst :: Ord a => [(a, b)] -> [(a, b)]
sortByFirst = sortBy (compare `on` fst)

the groupBy functions expect that the data is sorted, so we must first sort by each inner tuple (ie sort by (a, b) in ((a, b), [x,y,z]) )

groupByFirst :: Eq a => [(a, b)] -> [[(a, b)]]
groupByFirst = groupBy ((==) `on` fst)

then we use groupBy to group the resulting list of tuples into lists of lists of tuples, where each inner list shares the same first element.

squishByFirst :: (Eq a, Ord a) => [(a, b)] -> [[b]]
squishByFirst = (map . concatMap) snd . groupByFirst . sortByFirst

then we use some fancy mapping over snd to roll into each inner list, pull out the second element (the list), combine them together, and give back the result. This is equivalent to:

squishByFirst xs =
  let grouped = groupByFirst . sortByFirst $ xs
  in  [concat [ys | (_, ys) <- g] | g <- grouped]

---

You can also implement this as a fold over Map.insertWith

import Data.Map (elems, empty, insertWith)

squishByFirst :: Ord k => [(k, [v])] -> [[v]]
squishByFirst = elems . mkMap
  where
  mkMap = foldr (uncurry (insertWith (++))) empty

or for any Monoid:

squishByFirst :: (Ord k, Monoid vs) => [(k, vs)] -> [vs]
squishByFirst = elems . mkMap
  where
  mkMap = foldr (uncurry (insertWith mappend)) empty